Virus Classification: The World of Viruses

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Many viruses, infecting virtually all known life forms, have been discovered.

Virus classification is based on __. Why not genes?

molecular architecture, genetic relatedness, and host organisms

No genetic basis to classification because there is no gene that is shared among all viruses. There are just islands of similarity that they have.

Viruses are grouped into __.

Distinct naming conventions and classification schemes have developed in different domains of virology.

species, genera, and families

There are different ways to group and classify viruses, for example, by __ and __.

Many human viruses don’t bother with __ at all. They only have __. They appear to be similar to one another. They have different types of RNA genomes as well.

genetic material

whether or not it has an envelope

DNA

RNA

Study of the major groups of viruses leads to __

understanding of shared characteristics and replication pathways

Viruses with ssDNA genomes are __.
Viruses with dsDNA genomes include __.

small and have few genes

the largest known viruses

__ are very small.

Most plant viruses have __.

Many fungal viruses have __.

Most bacteriophages have __.

Many human diseases are caused by viruses with __.

The largest viruses (dsDNA) infect __.

ssDNA viruses

+RNA genomes

dsRNA genomes

dsDNA genomes

-RNA genomes

unicellular eukaryotes

Are viruses monophyletic?

Monophyletic means it shares a common ancestors

There are many arguments for and against.

For: they are more similar to each other than anything else

Against: You can make an argument against it. The fact that their genomes are so different

The evolutionary origin of viruses

1) The firs steps in the development of life on earth: __.

RNA world: viroids and RNA viruses may have originated in the RNA world

Explain the belief of age of viruses.

RNA viruses are believed to be older than DNA; they are the last remnants of the RNA world

RNA world

RNA is older than DNA; it was the first biological molecule; DNA and proteins and lipids came after that

RNA can contain its strucure, make copies of itself, and maintain enzymatic activity

Explain the transition to the DNA-based world.

DNA eventually evolved. It is not as stable as RNA. In our cells, there are also transposons, which move around.

DNA viruses may have evolved after the first viruses out of these enzymes that move around in our cells

Small and medium-sized DNA viruses could have arisen as independently replicating genetic elements in cells

Explain the largest viruses.

New viruses are changing our ideas of viruses

- They have dsDNA

- they have junk DNA

- they are very large, some as large as bacteria

- some of these viruses have degenerate genes that you only see in living organisms

- they have some genes for tRNAs, something that other viruses don't have; some genes for actual metabolism, changing biomolecules into others

Some of these really large viruses started off as __ or some other living parasite and sort of became __.

bacteria

more and more of a perfect parasite to the point where it stopped being alive

Baltimore classification of viruses

Viruses are characterized on type of genetic material

--> Everything comes back to positive sense RNA (similar to mRNA); it is the very last thing before proteins

They have to make proteins and DNA and wrap it together

The Baltimore classification, developed by David Baltimore, is a virus classification system that groups viruses into families, depending on their type of genome (DNA, RNA, single-stranded (ss), double-stranded (ds), etc.) and their method of replication.